1. Field of the Invention
This invention relates to a surface state inspection apparatus, and more particularly, to a surface state inspection apparatus which is suitable when detecting foreign particles, such as opaque dust particles or the like, other than a circuit pattern, adhered to a substrate, such as a reticle, a photomask or the like, in the semiconductor production process using a light exposure method.
2. Description of the Prior Art
In the IC (integrated circuit) production process, in general, an IC is produced by transferring a circuit pattern for exposure formed on a substrate, such as a reticle, a photomask or the like, to the surface of a wafer coated with a resist using a semiconductor printing apparatus (a stepper or a mask aligner).
At this time, if foreign particles, such as dust particles or the like, are present on the surface of the substrate, the foreign particles are also transferred in the transfer operation, causing a decrease in the yield of the IC production.
Particularly when a circuit pattern is repeatedly printed on the surface of a wafer by a step-and-repeat method using a reticle, one foreign particle on the surface of the reticle is printed on the entire surface of the wafer, causing a large decrease in the yield of the IC production.
Accordingly, it is indispensable to inspect for the presence of foreign particles on a substrate in the IC production process, and various kinds of inspection methods have been proposed. For example, FIG. 1 illustrates a method which utilizes the property of a foreign particle to isotropically scatter light. In FIG. 1, after being reflected by a mirror 511 for scanning and passing through a lens 512, a light beam from a laser 510 is directed upwardly or downwardly by interposing or withdrawing a mirror 513, and is incident upon the upper surface and the back surface of a substrate 515 reflected by two mirrors 514 and 545, respectively. The light beam scans the surfaces of the substrate 515 by rotating or vibrating the mirror 511 for scanning. A plurality of photosensing units 516, 517 and 518 are provided at positions separated from the optical paths of directly reflected light and transmitted light from the substrate 515. The presence of a foreign particle on the substrate 515 is detected using output signals from the plurality of photosensing units 516, 517 and 518.
That is, since diffracted light from a circuit pattern has a strong directional significance, output values from the respective photosensors are different from one another. If a light beam is incident upon a foreign particle, the incident light beam is isotropically scattered. As a result, output values from the plurality of photosensors become equal. Accordingly, by comparing the output values at this time, the presence of the foreign particle is detected.
FIG. 2 illustrates a method which utilizes the property of a foreign particle to disturb the polarizing characteristic of an incident light beam. In FIG. 2, the light beam from the laser 510 is made to have a predetermined polarization state via a polarizer 519, the mirror 511 for scanning and the lens 512, is directed upwardly or downwardly by interposing or withdrawing the mirror 513, and is incident upon the upper surface and the back surface of the substrate 515 by being reflected by the two mirrors 514 and 545, respectively. The light beam scans the substrate 515 with the mirror 511. Two photosensing units 521 and 523 having polarizers 520 an 522 in front thereof are provided at positions separated from the optical paths of directly reflected light and transmitted light from the substrate 515. A difference in the amount of received light due to a difference in the polarization rate between diffracted light from a circuit pattern and scattered light from a foreign particle is detected by the two photosensing units 521 and 523. The circuit pattern and the foreign particle on the substrate 515 are thereby discriminated.
However, the conventional methods have the following common disadvantages: That is, since inspection is performed by directly detecting light from an illuminated reticle by detectors, it is difficult to detect how a defect actually causing a problem in printing adversely influences a printed pattern. In the conventional methods, for example, a defect actually causing a problem in printing and a defect hardly influencing a printed pattern, that is, a defect which need not be detected, may be detected without being discrimated.